1. Technical Field
This disclosure relates generally to spiral inductors fabricated on silicon substrates, and more particularly to a spiral inductor structure with a substrate contact substantially adjacent to the spiral inductor and a predetermined doping distribution under the spiral inductor.
2. Description of the Related Art
Spiral inductors are widely used components of communication systems operating in the frequency range of 1 to 10 GHz and higher. At lower frequencies, inductive characteristics may be achieved with active circuits, while at higher frequencies, distributed LC components, such as microstrips, are utilized. Existing systems operating at radio and microwave frequencies; for example, frequencies greater than 1 GHz, use inductors fabricated on printed circuit boards, on quartz, or on GaAs substrates so that the carrier substrate has nearly ideal dielectric characteristics. Such inductor implementations may not have an electrical contact to the substrate, but instead have a solid groundplane on the backside of the substrate to contain the energy flow.
Recently, implementations of radio frequency (RF) and microwave transceivers on silicon substrates have been utilized to a greater degree due to the low-cost and high-volume manufacturing capabilities of silicon technology. Silicon is a relatively good substrate for RF and microwave applications, except for its low resistivity which causes high RF losses.
In the prior art, as shown in FIG. 1, a spiral inductor on a silicon substrate may be represented by a lumped-element model, in which the spiral coil 10 of the spiral inductor is modeled by an ideal inductance 12 labeled L.sub.S ; a series resistance 14, which may vary as a function of frequency f, that is, the series resistance 14 is R.sub.S (f); and an inter-wire capacitance 16 labeled C.sub.P between two ports 18, 20. The coupling of the spiral coil 10 to the substrate is represented by the oxide capacitances 22, 24 labeled C.sub.OX and the resistances 26, 28 labeled R.sub.B. A resistance 30 labeled R.sub.SUB is provided between the low-doped region under the spiral coil 10 and the substrate contact 32, which may be laterally spaced from the spiral coil 10. Two generic implementations of such an inductor in a circuit are known in the prior art: one-port implementations with one port of the spiral inductor connected to ground, and two-port implementations with both ports terminated at high impedance.
Further, two implementations of a substrate contact are known in the prior art; that is, the substrate contact may be substantially adjacent to the inductor so that the resistance R.sub.SUB is minimized, or the substrate contact may be omitted so that R.sub.SUB is treated as being infinite.
In the prior art, for example, in U.S. Pat. No. 5,481,131, which is incorporated herein by reference, spiral coils are known which are enclosed by metal rings without contacting such metal rings. Such metal rings may be a second plate of a capacitor and are not connected to the substrate.
In addition to the use of substrate contacts in the suppression of noise levels, substrate contacts provide improved operating characteristics in two-port implementations in which the ports 18, 20 of the inductor are terminated at high impedances. The inductance and Q may then be derived from the scattering parameter S21, which is the ratio of transmitted power at the output port to the injected power at the input port in two-port implementations.
Without a substrate contact, when the inductor is operated near the maximum Q, an input RF signal passes with a substantially identical magnitude through both the inductive path having the inductor L.sub.S 12 and the capacitive path having the capacitances 22, 24 labeled C.sub.OX ; that is, the signal across the capacitor C.sub.P 16 is typically negligible for operation near the maximum Q. With an ideal substrate contact, signal transfer via the capacitors 22, 24 labeled C.sub.OX to the output port is suppressed so that the maximum Q is limited mainly by R.sub.S, which represents ohmic losses in the spiral coil 10 and, to a minor degree, is limited by substrate losses.
Typically, there are two known methods for incorporating an inductor in a circuit: the ground potential is either common with one of the inductors ports or has a certain impedance between the inductor ports and the ground. Using a silicon substrate connected to ground, due to the intermediate resistivity of the silicon substrate, the associated ground potential is not well-defined equally across the length of the entire silicon substrate. Such uneven ground potential is due to the characteristics of bulk silicon in that bulk silicon is too low-resistive to be treated as a quasi-dielectric yet too high-resistive to be considered as a groundplane.
Additional doping of the silicon substrate may reduce the resistivity of the silicon substrate, but a high doping level under a spiral inductor causes relatively large eddy currents in the silicon substrate. As described in U.S. Pat. No. 5,446,311 and in pending U.S. patent application Ser. No. 08/594,455, now U.S. Pat. No. 5,656,849 issued Aug. 12, 1997 entitled "Two-Level Spiral Inductor Structure Having a High Inductance/Area Ratio", which are incorporated herein by reference, the occurrence of such relatively large eddy currents reduces the quality-factor (Q) of the inductor. For an inductor with a high Q, any doping is therefore generally blocked out from under the spiral coil.
Heretofore, such implementations of spiral inductors with high Q without contact to the substrate have encountered relatively poor operating characteristics due to such large eddy currents.